A numerical model is developed to study the performance of an integrated tubular fuel reformer and solid oxide fuel cell (SOFC) system. The model is used to study how the physical dimensions of the reformer, gas composition and the species flow rates of a methane feed stream undergoing autothermal reforming (ATR) affect the performance of an SOFC. The temperature in the reformer changes significantly due to the heat of reaction, and the reaction rates are very sensitive to the temperature and species concentrations. Therefore, it is necessary to couple the heat and mass transfer to accurately model the species conversion of the reformate stream. The reactions in the SOFC contribute much less to the temperature distribution than in the reformer and therefore the heat transfer in the SOFC is not necessary to model. A packed bed reactor is used to describe the reformer, where the chemical mechanism and kinetics are taken from the literature for nickel catalyst on a gamma alumina support. Heat transfer in the reformer’s gas and solid catalyst phases are coupled while the gas phase in the SOFC is at a uniform temperature. The SOFC gas species are modeled using a plug flow reactor. The models are in good agreement with experimental data. It is observed that the reformer temperature decreases with an increase in the reformer inlet water-to-fuel ratio and there is a slight decrease in the voltage of the SOFC from higher water content but an increase in limiting current density from a higher hydrogen production. The numerical results show that the flow rates and reformer length are critical design parameters because if not properly designed they can lead to incomplete conversion of the methane fuel to hydrogen in the reformer, which has the greatest impact on the SOFC performance in the integrated ATR reformer and SOFC system.