NOVELTY - The method involves introducing primary purification fuel gas (PG1) into multitubuler heat exchanger (20) for performing primary purification process. The fuel gas (BG1) is introduced into multitubuler heat exchanger (10) for performing secondary purification process in multitubuler heat exchanger (20). The primary purification fuel gas is flowed through tube (12a) of multitubuler heat exchanger (20) for flowing liquefied natural gas (NG1) between cylinder (20a) and tube of multitubuler heat exchanger (20) to perform secondary purification.    USE - Purification method of fuel gas from anaerobic-methane-fermentation and thermal decomposition of sewage and industrial wastewater, used in fuel gas refinery processing installation (claimed) such as fuel gas purification system.    ADVANTAGE - The primary purification fuel gas is flowed through tube of multitubuler heat exchanger for flowing liquefied natural gas between cylinder and tube of multitubuler heat exchanger, such that the resurgent gas adhering to the inside of multitubuler heat exchangers can be vaporized and can be supplied to each heat exchanger separately. The fuel gas purification functions of heat exchanger can be recovered by discharging the vaporized condensate from each heat exchanger outside. The energy for heating the resurgent gas can be reduced. The simplified structure can be achieved. The impurity concentration can be reduced. The operational efficiency of refinery processing installation can be improved effectively.    DETAILED DESCRIPTION - The method involves separating impurity contained in fuel gas by passing cooling-heat of liquefied natural gas through the fuel gas in a multitubuler heat exchanger by heating liquefied natural gas for purifying the fuel gas. The fuel gas is flowed between cylinder and tube of multitubuler heat exchanger (10) for flowing liquefied natural gas to perform primary purification. The unit purification time is planned in the impurity concentration of the primary purification fuel gas at secondary purification, so that the impurity concentration from purification fuel gas is flowed in tube of multitubuler heat exchanger (20). The inner flow path cross-sectional area of integrated tube is smaller than outer flow path cross-sectional area of flow path which is formed between cylinder and tube. The height of the fin in multitubuler heat exchanger is formed larger than the tube diameter. Multitubuler heat exchanger is equipped with several plain tubes whose center distance is more than outer diameter. The flow-path of primary purification fuel gas cross-sectional area of entrance side of tube is larger than flow-path cross-sectional area of exit side. The condensate is vaporized by resurgent gas (RG1,RG2) and is discharged from multitubuler heat exchangers. An INDEPENDENT CLAIM is included for refinery processing installation of fuel gas.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic view of the internal structure of the fuel gas purification system. (Drawing includes non-English language text)    Multitubuler heat exchangers (10,20)    Tube (12a)    Cylinder (20a)    Fuel gas (BG1)    Liquefied natural gas (NG1)    Primary purification fuel gas (PG1)    Resurgent gases (RG1,RG2)