The steam reforming of methane in a membrane reactor has been simulated with emphasis on the effect of hydrogen removal by the membrane on the poisoning effects due to traces of H2S and carbon deposition from the decomposition of methane. The performance of the membrane reactor under these conditions has been compared with that of fixed bed reactor operation for pressures up to 6 bars and temperatures in the range 773 to 873 K. Removal of hydrogen increases the tendency to poisoning from H2S and to carbon deposition. These effects can be reduced by operating at higher temperatures when H2S poisoning is dominant or by increased pressure for lessening the extent of carbon deposition