Abstarct: In this project, the utilization of combined heat and power systems and turbo-expanders in city gate stations is investigated technically and economically. Usually, high pressure natural gas loses its pressure in city gate stations by using expansion valves, to achieve to an appropriate and reliable pressure for consumption. But, in this expansion method, a considerable amount of pressure exergy of natural gas stream is ignored. Turboexpander is a reliable technology to convert this exergy to a useful form of energy such as mechanical or electrical energy. Because of increasing in heat requirements of station by using this system, it has been suggested to utilize a CHP system to meet the requirements of the station and to generate electricity as a peripheral product. Three different options with some different scenarios are introduced to utilize in CGS. A new algorithm is utilized to simulate and modeling of this options and scenarios. These options are utilization of just an expander in CGS, to utilize just a CHP system in CGS and the last case is to utilize both of them in typical city gate stations. Some city gate stations are investigated as case studies. These stations are some of Semnan province city gate stations. With regard to the discharge and the other parameters of the pressure reduction gas station , 6 stations in the Semnan province, was chosen for this study, that among these stations, Shahrood pressure reduction station with having the greatest discharge of the gas station, is the most suitable for this research and from the economic point of view is quite suitable for carrying proposed designs In total for all the 6 selected CGS stations and scenarios involving the use of gas turbines is also the amount of 29.66 MW electrical power can be produce which according to the selling price of electricity to the grid, the value of 17.4 million dollars electrical power can be sold to the national grid .