This work presents a simple and efficient method to produce a flat sheet composite membrane (FSCM) with minimum resistance in the support layer. The process is also low cost and uses a limited amount of solvent (water). In particular, a series of polydimethylsil oxane/low-density polyethylene (PDMS/LDPE) FSCM were produced by coating an active PDMS layer on a microporous LDPE support via continuous extrusion and salt leaching (68% wt. NaCl) using immersion in hot water (50 degrees C). The membranes were then characterized before and after leaching in terms of morphology, porosity, and pore size distribution, as well as thermal properties. The results showed that the microporous structure is highly correlated to the salt used and the amount leached out from the polymer structure. The resulting FSCM were finally used for gas separation of C3H8 from CO2, CH4, N-2 and H-2 in terms of permeability, solubility, and diffusivity. The results show that for an upstream pressure of 120 psi, the FSCM is about 6, 16, 23, and 53 times more permeable to C3H8 than CO2, CH4, H-2, and N-2, respectively, confirming the outstanding separation performance of these membranes for industrial applications.