Emission of nitrogen oxides (NOx) are one of the major environmental concerns arising from the combustion of syngas. Strategies to reduce emission and improve the efficiency of syngas combustion can be developed using computational fluid dynamic simulations to design cleaner and more efficient combustors. Toward this end, an accurate and efficient chemical kinetic mechanism that can describe the combustion chemistry of syngas with NOx under engine-relevant conditions is critical. In this work, a comprehensive survey of detailed mechanisms available in the literature for the syngas/NOx combustion reaction system is first conducted. A systematic and comparative chemical kinetic analysis of five detailed mechanisms is performed based on the reaction pathway and sensitivity analyses to identify the key reactions of the nitrogen species for a wide range of mixtures including the formation of NOx during syngas combustion and ignition of NH3, H2/N2O, and H2/NO2 mixtures. Comparisons of the reaction pathways from different detailed mechanisms indicate that the detailed chemistry is controlled by a small set of reactions and species. Recent high-level theoretical studies on HONO and HNO2 chemistry including previously neglected important reactions are updated. The rate constants for HNO + O2 = NO + HȮ2 are calculated using ab initio calculations in this work. An efficient high-fidelity skeletal mechanism consisting of 27 species and 130 reactions is developed based on a combination of the directed relation graph with the error propagation method and the simplified iterative screening and structure analysis method. Compared to the detailed mechanisms, the skeletal mechanism retains the major species and reactions for the syngas/NOx system and is validated against the typical experimental data, resulting in a very good performance.