In this work, a novel chemical modification is introduced to fabricate poly(ether-block-amide)/silica (PEBA/SiO2) nanocomposite membranes for the separation of CO2 from syngas and natural gas streams. cis-9-Octadecenoic acid (OA) was utilized for surface modification of the nanoparticles to restrict their agglomeration within the polymeric matrix. To our best knowledge, there is no evidence about the application of this modifier agent for the fabrication of nanocomposite membranes. The separation performance of fabricated membranes was investigated by pure and mixed gas permeation experiments. The incorporation of modified nanoparticles into the polymeric matrix improved the separation performance of the fabricated nanocomposite membranes. For instance, by increasing the loading content of the SiO2 nanoparticles from 0 wt % (the neat PEBA membrane) to 8 wt %, at 25 degrees C and 2 bar, the ideal selectivity values of CO2/H-2, CO2/CH4, and CO2/N-2 were improved from 9, 18, and 61 to 17, 45, and 137, respectively.