The paper studies numerically the thermal NO emission from a small vertical methane-fueled furnace supplied with the fuel through its primary inlet located at its bottom, and with the air via two opposing horizontal air jets perpendicular to the direction of fuel supply. Special emphasis is given to investigating the impact of radiation cooling due to the presence of soot in the flame established inside the furnace on NO emission from the furnace. Hereby two main issues are studied. These are the consequence of the commonly adopted assumption of decoupling soot from the gas phase computations, and the impact of the fuel/air reactivity level on NO concentrations. Results show that a complete decoupling of soot from the gas-phase computations leads, as compared with the case when soot and the gasphase are coupled, to a clear enhancement in soot levels, hence in radiation cooling which basically would depress NO formation. However, the difference in NO emission between the two cases is found to be small. Concerning the reactivity level of the fuel/oxidizer mixture, for the case when the reaction rate is reduced by a factor of 2 the overall amount of soot generated, hence radiation cooling, decreases noticeably. This, although reaction rates here are reduced, is responsible for the higher local temperatures and consequently the higher NO emission observed in this case, as compared with their corresponding levels when fuel/oxidizer reactivity levels increase.