Due to the nanometer scale pore size and extremely low permeability of a shale matrix, traditional Darcy’s law can not exactly describe the combined gas transport mechanisms of viscous flow and Knudsen diffusion. Three transport models modified by the Darcy equation with apparent permeability are used to describe the combined gas transport mechanisms in ultra-tight porous media, the result shows that Knudsen diffusion has a great impact on the gas transport and Darcy’s law cannot be used in a shale matrix with a pore diameter less than 1 μm. A single porosity model and a double porosity model with consideration of the combined gas transport mechanisms are developed to evaluate the influence of gas transport mechanisms and fracture parameters respectively on shale gas production. The numerical results show that the gas production predicted by Darcy’s law is lower than that predicted with consideration of Knudsen diffusion and the tighter the shale matrix, the greater difference of the gas production estimates. In addition, the numerical simulation results indicate that shale fractures have a great impact on shale gas production. Shale gas cannot be produced economically without fractures.