The article reported the fabrication of a new mixed-matrix membrane (MMM) made of nano- zeolitic imidazolate framework-7 (nZIF-7) or post-synthetically modified-nano-ZIF-7 (PSM-nZIF-7) blended within a polyetherimide (PEI) matrix. This work demonstrated the power of tuning the pore environments of MOF nanoparticles for drastically improving the permeability and selectivity properties of the MMM towards different gas systems. It is noted that the MMM fabricated from nZIF-7 exhibited the highest selectivities for CO2/CH4, CO2/C2H6, and CO2/C3H8 gas pairs. Furthermore, the MMM made of PSM-nZIF-7 displayed some of the highest permeabilities, which resulted in H2/CH4, N2/CH4, and H2/CO2 permselectivities that are remarkably well-positioned on the Robeson upper bound curves. This article considered a new insight for achieving superior membrane designs with previously overlooked blends. Finally, this work was a collaboration between Dr. Bassem’s team at KFUPM and Prof. Omar M. Yaghi’s group at UC Berkeley. The success of this research was the direct result of this collaboration, which has inspired further project development and the promise for more impactful discoveries to come out in the near future.