A polysulfone/polydimethylsiloxane (PSF/PDMS) composite membrane was developed by film casting of PDMS over polysulfone (PSF) support. The CO2/CH4 separation performance of developed membranes was assessed under dry and wet conditions to evaluate the influence of water swelling on the separation performance of the developed membrane. Gas permeance in developed pure and composite membranes was theoretically predicted considering the effect of swelling on gas permeation in the membrane. Swelling in the membrane was confirmed via comparison of the cross-sectional morphology of the developed membranes before and after wet tests. Water swelling in the membrane affected the separation performance of PSF, and CO2 permeance decreased from 48.79 to 2.37 GPU at 1000 kPa. However, the PSF/PDMS composite membrane resisted the effect of water swelling, and CO2 permeance increased from 39.9 to 40.87 GPU at 1000 kPa. CO2 and CH4 permeation were fitted with the dual mode model to predict the permeation in developed membranes theoretically. A modified dual mode model is considered with the best prediction of gas permeation under the effect of water swelling in the membrane.