Natural Gas (NG) processing is one of the major industrial separation processes. Membrane process, a relatively new technology among other available techniques, can be used for the removal of impurities like carbon dioxide from NG. Membrane performance has been described by different mathematical models over the decades. In this work, a simple mathematical model has been suggested to be incorporated with ASPEN HYSYS in order to design the membrane system for CO/CH, separation. Parameter sensitivities were analyzed by changing the operating conditions, such as feed composition and pressure and membrane properties (including selectivity of the membrane). Moreover, different configurations have been investigated for the optimized design including single stage (with and without recycle) and double stage membrane systems. It is shown that methane recovery can be improved by recycling permeate stream as well as by using double stage membrane system. © 2010 Asian Network for Scientific Information.