Zeolite particles, cross-linked poly(room-temperature ionic liquid)s (poly(RTIL)s), and room-temperature ionic liquids (RTILs) are combined to form mixed-matrix-membranes (MMMs) that exhibit a CO2/CH4 selectivity of 90 +/- 10 and a CO2 permeability of 260 +/- 20 barrers. This ideal (i.e., single-gas) separation performance is among the best reported in the literature, comparable to that of thermally rearranged polymers. The MMMs reported herein are physically robust and easily processed into membranes via conventional solvent-casting techniques. The elucidation and optimization of factors responsible for generating high-performance MMMs for CO2/CH4 separations were found to include: zeolite particle type, zeolite particle loading, poly(RTIL) structure, amount of cross-linking, and zeolite dispersion method in the RTIL. Optimization of these factors produced enhanced CO2/CH4 separation that significantly exceeds the 2008 Robeson upper bound, while the MMMs remain mechanically robust and easy to process. (C) 2016 Elsevier B.V. All rights reserved.