A coupled computational fluid dynamics and detailed kinetics study was performed to investigate the impact of finite mixing on the gas-phase partial oxidation of a gasifier outlet stream to selectively remove deposit precursors. The results showed that deposit precursors can be selectively removed as previously demonstrated on the basis of homogeneous gas mixture experiments and kinetic modeling. However, the finite mixing causes substantial degradation of the predicted performance. A detailed rate analysis showed that the imperfect mixing changes the kinetics of the gas-phase reactions. The reaction rates are much faster in the highly localized regions, where the O-2 concentration is high, and thus H-2 consumption by oxidation reactions become more competitive than H-2 regeneration. As a result, the imperfect mixing leads to a significantly less desirable outcome than the ideal mixing case. The performance can be improved by increasing the number air inlets to enhance the degree of mixing. The relative removal amount of deposit precursors can be increased if the initial O-2 concentration is increased, but it can cause substantial dilution by N-2 from air at the same time. Therefore, one should adjust the initial O-2 concentration depending upon the needs of a target system.