Industrial boilers of a particular type will behave differently due to manufacturing or assembly tolerances. In addition, the performance of a boiler will vary at different times of its service life. The product of this work is presented in a computer program designed to simulate a boiler unit. A method is presented that will allow the simulation to be adapted to a particular boiler's peculiarities by using modification factors and on-site measurements. Therefore, the model does not require details of boiler unit configurations and its conditions, and can be easily applied to different boilers. Performance of boiler heaters (i.e., superheaters, reheater, economizer, air preheaters) is simulated by the lumped parameter analysis. The model can be used to simulate fuel/gas-firing boiler unit performance at different times of its service life. The performance analysis includes (1) boiler unit first law efficiencies vs loads, (2) boiler unit second law efficiencies vs loads, (3) exit gas temperature, (4) performance analysis of superheater, reheater, economizer, and air preheater, and (5) approximate distribution of heat flux and temperature inside the boiler chamber. If the model is continually used in a boiler unit, maintenance or degradation analysis may be performed by analyzing the changes of modification factors.