In this study, we investigated the effect of tetra-n-butyl ammonium bromide (TBAB) on separation and/or collection of CO₂ from CO₂/H₂ (40:60) gas mixture via hydrate crystallization. The phase equilibrium conditions shifted to milder conditions as increasing the amount of TBAB additive up to 3.0 mol%. The existence of the critical concentration of the additive effects on the phase equilibrium conditions was also observed. The hydrate formation rate in 1.0mol% TBAB solution showed the highest value while the hydrate formation rate in 3.0mol% TBAB solution showed the lowest value. Raman analyses revealed that only CO₂ gas molecules enclathrated under experimental conditions carried out in this study. The phase equilibrium temperature of CO₂–H₂ (40:60) mixture gas hydrate systems with TBAB additive located in the range of between 283 and 290K at the integrated gasification combined cycle (IGCC) process pressure range of 2.5–5.0MPa. This study provides the potential of hydrate process to separate the CO₂ gas from CO₂–H₂ (40:60) mixture gas in IGCC process without significantly lowering the processing temperature.