A process simulation model of an externally reformed molten carbonate fuel cell (MCFC) system is used to analyze quantitatively parametric effects on system efficiency. In order to verify the MCFC process simulation model. a 25 kW system is analyzed on the basis of experimental data and its calculated efficiency is found to be reasonable. The overall system efficiency of a high-temperature fuel cell system, especially a MCFC, cannot be increased without proper thermal integration between the heat recovery units and without additional power from auxiliary power generation units such as turbines. The results of the simulation show that the configuration of the unit operators in a given system has a great effect on system efficiency, while system size and operating conditions have slightly less effects. Based on the system configuration, the optimal operating conditions (including fuel, oxidant utilization, and recycle ratio) can be specified to maximize the system efficiency.