The current study is part of a broader investigation, which deals with the application of a novel concept of integrated design to combustor modeling using computational fluid dynamic (CFD) methods. The main objective is the reduction of NOx emissions, The current work addresses the effects of mixing optimization and mixing model on NOx reduction during reburning Numerical simulations of the reburning process are conducted on a pilot-scale 0.5 MM Btu/h down-fired combustor (DFC)and compared with experimental data. Optimized mixing conditions resulted in a significant reduction,in NOx levels. A "reburn calculator" including the two-stage Lagrangian mixing model is used for predicting the NOx emissions in the DFC, and the results are compared with CFD-based simulations. Modeling results showed that for this case mixing is dominant over the chemistry, and hence the use of an advanced fluid dynamics model and a simplified chemistry scheme is adequate. Validation tests were conducted on optimized configurations using natural gas and coal water slurry as reburn fuels. The NOx reduction obtained with the optimized configurations was, as high as 75%, an almost 30% increase over the previous experiments on nonoptimized configurations, This study showed that an integrated approach, which includes mixing optimization, can be used effectively for a reburn case application, achieving significant NOx reduction; The effect of integration was demonstrated both theoretically and experimentally as applied to a pilot-sale DFC.