The rapid growth in energy consumption and environmental pollution have greatly stimulated the exploration and utilization of shale gas. The injection of gases such as CO₂, N₂, and their mixture is currently regarded as one of the most effective ways to enhance gas recovery from shale reservoirs. In this study, molecular simulations were conducted on a kaolinite–kerogen IID composite shale matrix to explore the displacement characteristics of CH₄ using different injection gases, including CO₂, N₂, and their mixture. The results show that when the injection pressure was lower than 10 MPa, increasing the injection pressure improved the displacement capacity of CH₄ by CO₂. Correspondingly, an increase of formation temperature also increased the displacement efficiency of CH₄, but an increase of pore size slightly increased this displacement efficiency. Moreover, it was found that when the proportion of CO₂ and N₂ was 1:1, the displacement efficiency of CH₄ was the highest, which proved that the simultaneous injection of CO₂ and N₂ had a synergistic effect on shale gas production. The results of this paper will provide guidance and reference for the displacement exploitation of shale gas by injection gases.