Abstract: A package boiler having water cooled surfaces positioned relatively close to the combustion burner to improve the radiant heat transfer. Extended surfaces are added to the water cooled tube fireside surface to increase absorption rates to the limits established by water side conditions to avoid departure from nucleate boiling (DNB). The heat release rates of multiple firing rate zone burners are matched to the absorption rates of the water cooled surfaces by adjusting the size and extent of the various firing rate zones. The burner is extended or the physical shape is altered to expose a larger surface to greater areas of water cooled furnace surface. The burner is matched to cooling surface to lower combustion zone temperatures and thus thermal NO.sub.X formations. Further, recirculated flue gas, excess air, water vapor or a combination of these may be used to add mass to the fluid stream flowing through the burner. Finally, either the gas fuel or combustion air may be staged.