In the present study, a comprehensive coarse-grained computational fluid dynamics and discrete element method (CFD-DEM) is developed for modeling the fluidized bed gasifier. Based on the model validations against experimental measurements, a series of simulations are conducted to investigate the effects of different operating parameters on the coal gasification in a bubbling fluidized bed. This study offers new insights into the effects of gas-solid mixing on the gasification reaction. The results show the non-uniform spatial distributions of the gasification rate in both the horizontal and vertical directions, which reflects the preferential distribution of fuel particles. It is found that the gas back-mixing plays an important role in controlling of the pyrolysis gas combustion, which is undesired for producing gasification products. The results show that the sand particle size has the most remarkable effect on the gas back-mixing. Though the uniformity of the fuel particle distribution is not a key factor influencing the average product yields, the horizontal fuel particle mixing is found to be the main cause of the fluctuations of the gasification rate. Fundamentally, it is demonstrated that the fluctuations of the horizontal fuel mixing are mainly dominated by the bubble behaviors.