Gas management has always been a challenging issue for mine operators, and this is becoming increasingly significant as the mining depth increases. Gas drainage sites or stubs are established to drill in-seam boreholes for pre-draining coal seam gas prior to gateroad development and longwall mining. The management of ventilation and gas emissions within the drainage site becomes a critical component of mine safety during gas drainage process. In this study, a three-dimensional (3D) computational fluid dynamics (CFD) model was developed based on an Australian in-seam drilling site to investigate the aerodynamics of seam gas (methane and carbon dioxide) emitting from the drilling site and boreholes during normal drilling, and in the case of a sudden gas inrush from the borehole. The model incorporates the major equipment within the drilling site and the common ventilation management practices (e.g., brattice and vent tube). Mesh independence studies were conducted to achieve a mesh independent solution. Initially, steady state calculations were conducted to analyze the effectiveness of different ventilation controls on gas removal, after which transient simulations were carried out to investigate the dynamic emission of gas from a borehole. Modeling results indicate that potential high risk zones can be formed if sufficient ventilation cannot be provided to the drainage site, particularly during a high gas inrush event from a borehole. The treatment of borehole discharges, the configuration of brattice as well as the layout of ventilation tubes all played important role in effective gas management. This study has demonstrated that CFD modeling technique can be a useful tool for the design of optimum ventilation/gas management in underground gas drainage site in coal mines, especially when abnormal gas emission is encountered during drainage operations.