Abstract: A method for combusting fuel in a fired heater having one or more burners, the method. comprising: (a) combusting at least a portion of the fuel in the. presence of an oxidizer in the burners, generating heat and producing a flue gas; and. (b) recycling at least a portion. of the flue gas to. the burners; wherein step (a) and step (b) are conducted in a manner such that the combusting takes place in a combustion regime represented. by 4,500?7554.8?933.72x+64.960x2+0.47705y?0.55680z1579.22,500; W wherein w represents a mole fraction of oxygen. in the oxidizer, x represents a recycle ratio of the flue as measured in moles of flue gas/mole of oxidizer; wherein y represents a temperature of the. flue gas in Fahrenheit recycled to the burners; and wherein. z represents a heating. value of the fuel in Btu/scf LHV.