The increasing trend of complexity in the power system due to the utilization of the vast number of options in order to support the increasing consumption of the electricity and improve the efficiency level , needs new procedures to improve the performance of the power system from the economic and the technical aspects . The challenge in the power system planning is attaining the optimal economic performance while the technical performance is acceptably improved . In order to achieve to this goal , in this thesis , we proposed a model to obtain an economically optimal performance of the power system through an optimize coordination of the power system flexibility options . To achieve this aim , we presented a framework to coordinate the power system flexibility options including the allocation and utilization of the electrical energy storage systems , implementation of the demand response programs , consideration of the transmission switching possibility and also utilization of fast responsive power generation units with higher ramp rate constraints in the power systems to enable the system to adapt to the different operational conditions . Accordingly , as the first step of our proposed modeling procedure for the power system planning , we tried to introduce an optimal planning method associated with the coordinated consideration of the power system flexibility options . Afterward , as the next step we studied the impacts of the natural gas network constraints and limitations on the performance of the power system . This approach has been adopted in order to analyze the interdependency between the power system and the natural gas network . To generalize the proposed model of coordinated optimization of the power system flexibility options considering the natural gas network constraints , aforementioned two networks are considered as two parts of a greater system with a considerable impact on the each other . At this step , we were aimed at improving the performance of the entire system from the economic and technical aspects . As the last step of modeling , the proposed model is enhanced to a stochastic coordinated optimization problem , able to encounter with the uncertainties of wind power generation . The coordinated optimization problem for the power system flexibility options is modeled in the context of a Mixed Integer Linear Programming (MILP) . In order to solve the large scale optimization problem , proposed in this model , we formulated the problem in the form of the Benders Decomposition algorithm . The simulations of the proposed model implemented on the numerical case studies have been solved by GAMS v.24.1.3 using the solver CPLEX v.12 . The numerical case studies of this thesis are conducted on modified versions of the IEEE 14-bus benchmark and the Belgian 20-node high-calorific gas network , adapted for the numerical case studies of this thesis . The analysis conducted on the results of the proposed model implementation on the studied cases , acknowledge the efficiency of the proposed model at each step of the study on the power system economically optimal performance and the flexibility improvement , simultaneously , under different operational conditions . The improvement in the performance of the entire system from the technical and the environmental aspects is also among the noticeable achievements of this study . The results of this study , due to the adoption of a holistic approach may help the power system and the natural gas network operators and decision makers adopt more economically efficient procedures in order to improve the operational status and issues pertaining to the entire system .