Synergistic development of energy by combining geothermal and hydrocarbon gains an increasing attention. In practice, hydrocarbon reservoir fluids produced from oil and gas wells are used for electricity generation through a binary cycle power plant. To enhance the thermal recovery efficiency, we present an innovative design of in-situ geothermal power generation and illustrate a promising method of geothermal energy utilization in a cost-effective and environmentally friendly manner. This proposed design is an integration of thermoelectric generation technology, well completion and production operation. In this design, electricity is generated downhole by thermoelectric generators (TEG) installed on the outer side of the production string to convert the thermal energy to electric power. Heat transfer model is set up to determine the temperature field with a case study to demonstrate the downhole power generation efficiency. The results are compared with both simulation and experiment results. The proposed design capitalizes on existing hydrocarbon production wells and retrofit them for power generation. This practice can be a great complementary for surface power plants, and could reduce operational cost and extend the economic lifetime of oil and gas wells, especially for high water-cut wells in mature fields.