To save natural gas (NG) in heating steel slabs, a heat-recovery device (HRD) is used in a metallurgical pusher furnace together with refuse blast-furnace gas (RBFG) and O-2. These NG-saving techniques are evaluated by using finite-time thermodynamics; The results indicate that mixing a higher percentage of RBFG with NG does not necessarily save NG. However, a higher percentage of O-2 in air and a larger HRD do save NG.