Various efforts have been explored to save the cost in many industrial fields. In order to recover the residual thermal energy from the flue gas, an extreme high efficiency heat exchanger is planning to install at a power plant. The gas temperature will be reduced to $40^{circ}C$ from $115^{circ}C$. Thus gas buoyancy decreases, and dispersion of nitrogen oxides is expected to deteriorate as increasing relative humidity. In this study, the conversion of nitrogen monoxide to nitrogen dioxide and dispersion regime are investigated through computational modeling. Nitrogen dioxide which indicates 0.1 ppm at 85 m from the ground could be propagated to 620 m at $115^{circ}C$ of the flue gas, whilst when cooled down to $40^{circ}C$, it expands up to 750 m. The ground level influence area showed more expansion of dispersion, approximately to 930 m.