The paper discusses modelling of coupled heat transfer and electricity generation in a thermoelectric generator designed for reliable island-mode power supply. The considered generator is a new concept of a low power supply (50 W) whose aim is to provide electricity for remote gas pressure reduction stations with the purpose to maintain the control and automation equipment. This equipment contributes to the system safety and minimizes the risk of unintended methane emissions. The thermoelectric generator is designed for reliable and maintenance-free operation and power supply. Natural gas is burned in a partially premixed burner and the flue gas heats the hot side of the thermoelectric generator. The combustion air cools the cold side of the thermoelectric generator, providing the temperature difference required for electricity generation occurring based on the Seebeck effect. The flow of air and flue gas through the system is driven by chimney draft. The developed model couples the heat transfer on the hot and the cold side, as well as the generation of electrical energy inside the thermoelectric modules. The model takes into account convection and conduction in the gas flow conduits and in finned heat exchangers of the cold and hot sides. The analysis demonstrates the relevance of design and operational parameters on the boundary temperatures of the thermoelectric modules. The obtained results will be used in further (ongoing) phase leading to the design and construction of a prototype electricity generator dedicated for island-mode supply.