A significantly improved defect-free ester-crosslinkable hollow fiber membrane was developed through simultaneous optimization of the spinning solution and dry-jet/wet-quench spinning process variables. The resultant ester-crosslinked hollow fibers show extremely high separation productivity and efficacy with CO2 plasticization resistance under aggressive feed conditions. The CO2 permeance was improved from 50 GPU to 117 GPU with a CO2/CH4 selectivity of 37 in testing at 200 psi with 50/50 CO2/CH4 feed, 35 degrees C. Moreover, the crosslinked hollow fibers maintain a high CO2 permeance under highly aggressive feed pressures up to 800 psi of 50/50 CO2/CH4 feed without CO2 plasticization. A lower operating temperature can improve the CO2/CH4 selectivity significantly without apparent loss of CO2 permeance. The mixed gas permeation with high CO2 feed pressure demonstrates that ester-crosslinked hollow fiber membranes provide a significant advance in the state of the art for CO2/CH4 separations. (C) 2012 Elsevier B.V. All rights reserved.