The experimentally observed rise in the single pass methanol production with CO2 and H-2 (T = 200 degreesC, P = 4.3 bar) by in situ product removal over a perm-selective Nafion (R) membrane is verified by model simulations. For this purpose, software was developed which addresses the two rivalling processes (synthesis and permeation) for given catalyst activity and permeation performance of the membrane. The program predicts the production rate of all the reactor gas components leaving the reactor by explicit integration of the gas component and (reactor and mantle) space specific differential rate equations along the symmetry axes of the tubular membrane reactor. The activity of the CO2-tolerant catalyst used was characterized independently by kinetic model analysis; the experimentally determined permeation performance of the membrane for the various reactant gas components by Arrhenius type temperature dependencies. The model confirmed the membrane reactor results satisfactorily well. It was estimated that with 10 mum thin membrane surfaces implemented in commercial methanol synthesis plants and operated under technically relevant conditions (T = 200 degreesC, P = 40 bar, GHSV = 5000 h(-1)), the single pass reactor yield improves by 40% and that the additional costs for the membrane material are two production months only. (C) 2001 Elsevier Science B.V. All rights reserved.