Industrial-scale gasifiers are traditionally designed using performance data from small-scale systems and expert experiences. In this research, the computational fluid dynamics (CFD)-coarse-grained discrete element method (CGDEM) model in open-source software MFiX was employed to assist the design of a 22 MW updraft moving-bed coal gasifier. A quasi-one-dimensional CFD-DEM simulation was first performed; its results were then served as an accurate initial condition for the three-dimensional (3D) CFD-CGDEM simulation. Then, long-time 3D simulation of 3.89 h of physical time using 120 CPU cores was achieved for the industrial-scale reactor at a quasi-steady state. Important results were obtained such as syngas compositions, gas flows, solid and gas temperatures inside the reactor, ash distribution, and zone locations. The high-fidelity CFD results compare well with experience-based design from Hamilton Maurer International, Inc. Apart from predicting the steady-state operations, several transient operations including intermittent coal feed, turndown, shutdown, and accident load loss were investigated. The results showed that syngas components share the same cycles in intermittent coal feed conditions. There is a decrease in H2 and CO and an increase in tar when the gasifier operates at a lower capacity. The shutdown process took more than 4.4 h and the recovery from a sudden loss of the bed material took more than 2 h. These studies provide helpful guidance for industrial-scale reactor design and rehearsal of potential risks in real-life operations.