A mathematical model taking into account mass dispersion is presented for the simulation of the performance of an adiabatic full-scale membrane reactor. Results are presented from the application of this model to a membrane reactor, that is introduced in an integrated gasification combined cycle plant, to control carbon dioxide emissions. The performance of this reactor equipped with highly selective membranes is studied in detail. The extent of the impact of the non-ideal flow effects on the membrane reactor operation is discussed. It is shown that dispersion effects have a negative influence on the performance of industrial-scale membrane reactors and they must be considered as a basic parameter to their design.