High-performance ester-cross-linkable composite hollow fibers were developed through simultaneous spinning of 4,4'-(hexafluoroisopropylidene) diphthalic anhydride (6FDA) based polyimide and Torlon (registered trademark of Solvay Polymers) solutions. The core layer polyamide-imide polymer, Torlon, demonstrates excellent adhesion with cross-linkable 6FDA-based polyimide during spinning and ester-cross-linking. Torlon also shows superior thermal stability after cross-linking while maintaining the open substructure of the Torlon core layer. The resultant cross-linked composite hollow fibers show a CO2 permeance of 40 GPU with a CO2/CH4 selectivity of 39 in testing at 100 psi with 50/50 CO2/CH4 feed, 35 degrees C. The cross-linked composite hollow fibers also show high separation performance in the presence of high-level toluene contaminants up to 1000 ppm, suggesting that cross-linked composite hollow fibers are viable under realistic operation conditions. The high natural gas separation performance and significantly reduced cost of hollow fiber formation provides a significant advancement in the state of the art for natural gas separations.