In this work, the performance of the Fischer–Tropsch reaction over iron catalyst in three reactor configurations was theoretically studied and analyzed. Indeed, the control and the guidance of produced hydrocarbons composition were done using a new concept of membrane separation reactor. Our findings show that the composition can be imposed by the nature of the integrated membrane. Depending on the used membrane, the water gas shift reaction equilibrium can be shifted towards hydrogen production or towards carbon monoxide production and consequently, the H2/CO ratio was changed in situ for giving olefins or paraffin according to our requirements specification. Finally, it was found that using a water permselective membrane can boost the composition to C3–C5 olefin compounds, whereas the separation of carbon dioxide can enhance the formation of paraffins.