A natural-gas fired walking hearth type furnace for the reheating of steel billets was investigated in this work. A novel numerical approach was used to predict the gas phase combustion, heat transfer and transient heating characteristics of the billets in the furnace. In contrast to conventional coupled simulation of the heat transfer and the heating characteristics, the iterative solution procedure used considers steady-state gas phase combustion of the furnace and unsteady simulation of the billets separately. Due to the steady flamelet model for combustion modelling the calculation time was kept to a minimum although a detailed reaction mechanism with 17 species and 25 reversible reactions was used. The reaction mechanism is applicable for different combustion environments and can be used for future investigations of the furnace after adaption for oxygen enriched combustion. Burner geometry was modelled in detail for a high accuracy prediction of the flame shape of the flat flame burners as well as temperature and species concentrations in the vicinity of the burner. The simulation revealed that 93% of the total heat flux to the billets was contributed by radiation. Transient heating showed a maximum temperature difference inside the billets in the heating zone at 192 K decreasing to a value between 42 and 49 K at the end of the furnace. (C) 2015 Elsevier Ltd. All rights reserved.