For unconventional reservoirs, the effect of adsorption on phase equilibrium cannot be neglected because the process occurs in extremely tight porous media. This work focuses on the adsorption prediction of multicomponent mixture systems in tight porous media with the oil-gas-adsorption three-phase equilibrium model to a gas sample in the literature. The revisited vacancy solution model of adsorption by Bhatia and Ding is introduced to study the adsorption behaviors of the mixture. The gas and adsorbed phases are assumed to be the solutions of adsorbates with a hypothetical solvent called "vacancy", and the vacancy is treated as an additional component engaged in the phase equilibrium in this theory. Instead of using parameters extracted from the multicomponent adsorption data, this method takes advantage because it accurately predicts the gas mixture adsorption equilibrium with consideration of non-ideal behavior in the adsorbed phase from pure gas adsorption isotherms over wide ranges of conditions, which could be efficient in terms of cost and time. It can explain the competitive adsorption phenomenon, which is proven during the adsorption process of the gas mixture. The experimental data in the literature of CH4-C2H6 binary gas mixtures of different compositions with a pressure ranging from 0 to 125 bar under the temperatures of 40, 50, and 60 degrees C are restudied in this work. The prediction results are compared to two other methods, including the extended Langmuir model and the multicomponent potential theory. This method shows an improved precision with less than 5% mean absolute percentage error in all cases. In addition to predications of desorption for the depletion process, the vacancy solution model has the potential in future work to give simulations for other production operations, such as CO2 or N-2 injection for the displacement of hydrocarbons in shales.