Ionic liquid modified Pebax 1657 thin-film composite (TFC) membranes were synthesized on different porous supports for improved separation of CO2 from CH4 and N2. XRD, SEM, FTIR, TGA, DFT, and BJH tests were utilized for the characterization of TFC membranes and supports. The incorporation of 1,3-Di-n-butyl-2-methylimidazolium chloride ionic liquid into the Pebax 1657 solution led to better compatibility between selective layer and support. The results of gas permeation tests showed an enhanced CO2/CH4 and CO2/N2 selectivity for most of the ionic liquid modified TFC membranes. To derive a predictive model for the permeability of gases in TFC membranes, correction factors (β) for fractional free volumes of TFC membranes were linearly correlated against correction factors (α) for porosity and tortuosity of the substrate in the dusty gas model using CO2 experimental permeance data at 20, 60, 100 and 140 °C. The resulting modified model showed remarkable effectiveness in the prediction of permeability of CH4 and N2 in the TFC membrane at investigated temperatures. The comparison of model predictive performance with previous models showed the supremacy of the new model in terms of average absolute relative errors (AARE) and the standard deviation of relative errors (σ).