Mathematical modeling of top fired steam reforming reactor and its furnace is presented. For the reactor side one-dimensional heterogeneous model with mass transfer limitation in catalyst pellets was considered. A three-dimensional zonal analysis was used in the furnace side for heat transfer modeling. Results from literature and data from an industrial top fired steam reformer were used to validate the model. Effects of important design and process parameters were studied using the combined model. A maximum temperature in external tube skin was observed in one third of the reactor length from top. It was shown that a 20% rise in effective emissivity of tubes leads to an increase of approximately 3.5% in reactor exit temperature. In addition reduction of extinction coefficients of combustion gases by 25% leads to about 2.6% rise in temperature. (c) 0 2007 Elsevier B.V. All rights reserved.