A thermoelectric energy conversion system could realize a direct conversion between thermal and electrical energy. The thermoelectric module composes a thermal resistance network with the hot and cold side heat exchangers within an integrated thermoelectric system. The heat transfer between the network and the heat source/sink is critical to the system performance. In this paper, we analyze the effect of external heat transfer on the system internal temperature distribution based on the coupled heat transfer relationship among different nodes in the thermal resistance network, and derive a set of analytical formulae of the system performance in practical thermal environments. Furthermore, a direct relationship between the external heat transfer conditions and the peak output power of the system is uncovered, and a general strategy for quickly designing thermoelectric energy conversion systems is proposed.