The present study is an attempt to investigate the effects of preparation and operation parameters on ideal and real selectivity of polydimethylsiloxane (PDMS) coated asymmetric polyethersulfone (PES) membranes for natural gas sweetening. Scanning electron microscopy (SEM) was used to study the effect of PES concentration and solvent type on membrane morphology. The effects of operating parameters on the performance of membranes were investigated by using binary CO2/CH4, H2S/CH4 and ternary H2S/CO2/CH4 gas mixtures. Higher concentrations of PES and a higher sequential coating number increase CO2/CH4 selectivity and decrease CO2 permeance. As a notable and interesting result, the membrane exhibits rubbery PDMS behaviour for H2S containing feeds and displays a glassy PES membrane for CO2/CH4 mixed gas. An increase in the feed pressure decreases CO2 and CH4 permeance and increases CO2/CH4 selectivity for CO2/CH4 feed. For H2S/CH4 and H2S/CO2/CH4 mixed gas, enhancing the feed pressure results in higher CH4 and lower CO2 and H2S permeance and a declined CO2/CH4 and H2S/CH4 selectivity. Increasing temperature in binary CO2/CH4 enhances CO2 and CH4 permeance and decreases CO2/CH4 selectivity. Increasing the temperature increases CH4 permeance and decreases H2S permeance for binary H2S/CH4 mixture. For ternary mixture, increasing the temperature leads to a higher permeance for CO2 and H2S and a lower CH4 permeance. For binary CO2/CH4, a higher CO2 concentration increases the membrane gas permeance and decreases CO2/CH4 selectivity. Increasing the H2S concentration in the feed results in a reduction in gas pressure normalised flux of gases in ternary gas feed because of an increase in Flory-Huggins interaction parameter.