Experimental studies were carried out to separate CO2 from gas mixtures of CO2-N-2, using a small polymer membrane module. When the feed gas is a mixture saturated with water vapor at 50 degrees C, the permeate stream always shows higher composition ratio of CO2/N-2, compared with the dry mixtures as a feed. Computer simulations can explain the experimental data. Water vapor whose permeability is the largest in this mixture permeates first through the membrane, resulting to dilute CO2 concentration in the permeate stream. Water vapor in the gas mixtures causes the membrane system to maintain a larger driving force for CO2 transportation than a dry mixture feed. When the wet mixture is fed, the permeate of cocurrent flow pattern shows the higher composition ratio CO2/N-2 than those of the countercurrent flow pattern up to the moderate range of CO2 recovery. The simulation study can explain these phenomena. The permeate stream is diluted by the water vapor over whole zone in the cocurrent flow pattern, whereas water vapor exists in a narrow region in the countercurrent flow pattern. Consequently, the cocurrent flow mode generates a wider zone of large CO2 driving force than the countercurrent mode. In the case of cocurrent flow, the non-dehumidification process of membrane separation method combined with liquefaction has an economical advantage in operation energy compared to the hydration process of membrane separation method combined with liquefaction. But in the case of countercurrent flow, the opposite result is found.