This study supports the development of a 20-kW(e) biogas reformer for PEMFC applications. Biogas contains a high concentration of carbon dioxide (CO2) in the feedstock, which can act as a reforming agent but increases the carbon monoxide (CO) concentration in the product. The feedstock composition and the external heat supply impact both the methane (CH4) conversion and the CO concentration in the product; thus, a numerical model is developed to investigate these impacts. As the CO2 content in the feedstock or the external heat supply increases, both the CH4 conversion and the CO concentration in the product increase. A high CO2 concentration or a high operating temperature stimulates the reforming reactions but also shifts the equilibrium to favor CO production. However, the performance can be improved by controlling the heat supply method. Using a more uniform temperature distribution at approximately 650 degrees C, the CO concentration decreases while the CH4 conversion increases. This finding suggests the importance of temperature control in biogas steam reforming. Copyright (C) 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.