The reforming units are basically furnaces containing burners (which provide a large amount of heat by fuel combustion) and tubes packed with supported nickel catalyst. Due to the high heat input through the reformer tube wall and the endothermic reforming reactions, the catalyst tubes are exposed to significant axial and radial temperature gradients. For this reason, a two-dimensional mathematical model that takes into account the diffusion reaction phenomena inside the particles rigorously has been used to represent the reactor. Strong radial temperature gradients in the reformer tube have been found, particularly close to the reactor entrance. These temperature differences cause significant variations in the methane reaction rate along the radial position, being the catalyst close to the reforming tube center poorly used. For this reason, the reforming tube diameter and the catalyst activity distribution were modified to use the catalyst more efficiently. The tube diameter has an important influence on the reformer performance, considerable higher conversions and reactor capacities per tube (i.e. closer equilibrium approaches) have been observed for the tubes with smaller diameters. The catalyst activity distribution also strongly impacts the reactor operation. The use of two catalysts of different activity, adequately distributed along the axial and radial directions, allowed to significantly decrease the maximum tube wall temperature and simultaneously minimize the reactor volume fraction packed with the catalyst of higher activity. (C) 2003 Elsevier Science B.V. All rights reserved.