Raw and tetraethylene pentamine (TEPA) functionalized titanium oxide nanotubes (TiNTs) were used as filler particles to prepare nanocomposite membranes. Polyethylene glycol (PEG) was blended with the Pebax-1657 as matrix and incorporated by TiNT-TEPA to prepare Pebax-1657/PEG/TiNT-TEPA nanocomposite membranes based on Response Surface and Central Composite Design methods of experimental design. The TiNT-TEPA and the Pebax/PEG/TiNT-TEPA were characterized using TEM, FESEM, SEM, FTIR or ATR-FTIR, XRD (EDS) and mechanical strength analysis. CO2 adsorption capacity of TiNTs and TiNT-TEPA-70% were found as 0.71 and 4.2 mmol/g, respectively. CO2 permeability and ideal selectivity of optimum (in term of separation performance) membrane, as Pebax (7 wt.%)/PEG (7.5 wt.%)/TiNT-TEPA (3 wt.%), were increased by 67.7 and 11.7%, respectively, compared with the neat Pebax membrane. The optimum membrane's CO2 permeability and CO2/CH4 ideal selectivity were investigated at operating temperatures of 20-45 degrees C and reveled 18.5 and 9% changes, respectively. Its transmembrane pressures increment form 4 to 10 bar for single and CO2 (50)/CH4 (50) mixed gas resulted in CO2 permeability and CO2/CH4 ideal selectivity enhancements by 19 and 20%, respectively. The optimum membranes' mixed gas separation performance were found lower than that of single pure gases at the same operating condition. (C) 2019 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.