NOVELTY - An excess steam (S0) is added to waste gas (G0) containing carbon monoxide and shift reaction is performed. A mixed gas (G) containing hydrogen and carbon dioxide produced by the shift reaction, and steam which is not consumed by the shift reaction, is contacted with an organic substance, to modify and to reduce molecular weight of the organic substance. The combustion heat of gaseous fuel generated by molecular weight reduction process is controlled by total flow volume of waste gas (G0) and excess steam (S0), in the molecular weight reduction of organic substance.    USE - Reduction of molecular weight of organic substance e.g. plastics, oil-impregnated sludge, waste oil and biomass (all claimed) into gaseous fuel.    ADVANTAGE - The method enables efficient reduction of molecular weight of organic substance and generation of gaseous fuel with controlled combustion heat, by simple process, and using economical installation. Recycling of organic substance is enabled with suppressed generation of carbon dioxide, by reforming carbon dioxide generated by the shift reaction to carbon monoxide. Suppression of emission of carbon dioxide and manufacturing cost of iron is enabled by reducing the usage amount of coke in the operation of blast furnace.    DETAILED DESCRIPTION - An excess steam (S0) is added to waste gas (G0) containing carbon monoxide generated from a metallurgical furnace, and shift reaction is performed. The mixed gas (G) containing hydrogen and carbon dioxide gas produced by the shift reaction, and steam which is not consumed by the shift reaction, is contacted with an organic substance, to modify and to reduce molecular weight of the organic substance. The combustion heat of gaseous fuel generated by molecular weight reduction of organic substance is controlled by total flow volume of the waste gas (G0) supplied to a shift reactor (1) which performs the shift reaction, and excess steam (S0), in the molecular weight reduction of the organic substance. INDEPENDENT CLAIMS are included for the following:    (1) operation of blast furnace, which involves blowing the gas product generated by the molecular weight reduction of the organic substance into the furnace; and    (2) system for molecular weight reduction of the organic substance, which has the shift reactor, reformer (2) in which reduction of molecular weight of the organic substance is performed, and control unit (7) which controls the combustion heat of the gaseous fuel generated by the reformer.    DESCRIPTION OF DRAWING(S) - The drawing shows the explanatory view of the system used for reduction of molecular weight of organic substance. (Drawing includes non-English language text)    Shift reactor (1)    Reformer (2)    Fluidized bed molecular weight reduction furnace (2x)    Extraction unit (3)    Rotary kiln combustion furnace (4)    Supply unit (5)    Separation unit (6)    Control unit (7)