Aiming at improving the quality of syngas, the thermochemical behavior of syngas formation during a single coal particle gasification process is investigated based on a validated numerical model. Initially, simulations of coal gasification with steam (H2O) are conducted in a reactor, and the results show that the steam gasification generally favors the production of H-2 and CH4. However, the switch of agent to CO2 into the gasifier influences the gasification products having H-2, CO, and CH4. This then prompts the investigation of a mixture of H2O and CO2 agent in the reactor's environment, and the results show a promising indicator in producing an overall better syngas quality. Moreover, the influence of the coal particle size and gasification temperature on the syngas production is studied. The results identify that the concentration of syngas products is higher when using smaller coal particles as the behavior of heterogeneous reactions of CO formation is affected by the particle size. Finally, high temperature promotes the chemical reactions of the gasification process, resulting in the improved production of syngas.