A CFD-based integrative approach for combustor design is presented and applied to a pilot-scale 147 kW down-fired combustor (DFC) unit for maximum NOx reduction. The approach is based on synergistic integration of several NOx control methods, including burner optimization, air staging, and reburning. The performance of coal, coal-water slurry, and biomass as reburn fuels was predicted using numerical simulations and compared with measurements. Reduction of NOx levels was targeted at every stage, the results being coupled with the optimized parameters for mixing and injection configuration. A sensitivity analysis was conducted in order to evaluate the variation of predictions with respect to model parameters. Modeling results show that improved mixing and burner optimization can contribute significantly to lowering the primary zone NOx levels. Validation results for natural-gas and coal-water-slurry reburn on optimized injection configurations are included and show important NOx emissions reduction. Different scenarios are discussed and recommendations are made for maximum NOx-reduction efficiency.