Methane steam reforming is the most important industrial route to produce H-2. The process is governed by equilibrium reactions, the overall process is endothermic and high temperatures are required to reach satisfactory methane conversions. The possibility of using a membrane reactor, which separates H-2 from the reaction zone with a subsequent improvement of the conversions, is a challenge of many academic and industrial researchers. A great effort of membrane reactor analysis applied to steam reforming is necessary in the light of the novel and potential process applications (fuel cells, CO2 capture). This paper presents the model of a non-adiabatic methane steam reformer membrane reactor (MSRMR) working in equilibrium conditions. The model was used to investigate the effects of some variables (e.g. temperature profile, separation efficiency, plant size) on the membrane area and the energy required by the process, which in turn affect fixed and operating costs. The simulations showed that the membrane area required sharp increases in the reactor size and that for large plants the development of thin and permeable membranes is a key issue. (c) 2006 Elsevier B.V. All rights reserved.