A numerical simulation for analysis and optimization of the performance and NOx production was applied to a natural gas fuel boiler in South Pars Gas Complex. For this purpose, nine actual environmental and operational parameters of a boiler were measured and recorded every two hours and then averaged daily for a year. Using the thermodynamic laws, important parameters such as body and exhaust flue gas losses, as well as the thermal efficiency and exergy efficiencies of the combustor and boiler, were calculated for each day. The results show that, owing to changes in the environmental and operational conditions, the thermal and exergy efficiency of the boiler varied from 83% to 87% and 27% to 32%, respectively, during the year. In addition, by optimizing the excess air percentage, the thermal and exergy efficiencies could be increased by 1.5% and 3%, respectively, for most days of the year.