A 3-D computational fluid-dynamics model containing equations for mass, heat and momentum transfer and using a k-epsilon closure model, was used to calculate the 3-D flue-gas flow pattern and the corresponding 3-D temperature field for a pyrolysis furnace. The computational fluid dynamics model is combined with a reactor model for the cracking tubes and a radiation model for the radiative heat transfer in the furnace box. Detailed reaction kinetics for the naphtha cracking reactions in the tubes and a five-step reaction mechanism for the combustion of methane in the flames were used. This complete model reveals asymmetric flow patterns in a naphtha-cracking furnace with 4/2/1 split-coil reactors and fired with long-flame burners.