In this paper the results of a study on reducing or recovering of the flare gases of the olefin unit of the Amirkabir petrochemical complex in southwest of Iran is reported. In particular, the focus of the paper is on the mass flowrate measurement of flare gases (mainly H2 and CH4) which is not an easy task in units without required devices installed. Since, generally, the flare gases are not the first priority in the unit operation, and the costs involved, usually these devices do not exist. Considering the characteristic of flare gases, the available portable measurement devices are unable to reliably measure the flare gas mass flowrate, and therefore, indirect or soft-sensing approach was used. Three indirect approaches were used to ensure the accuracy of the results. It was concluded that flare gas recovery unit is not needed; meanwhile, four scenarios were introduced to stop burning about 10000 kg/hr flare gas and techno-economical analysis were performed to rank them. The direct profit could be as much as ten million dollars annually, in addition to preventing of the discharge of a considerable amount of harmful burned gases to the atmosphere.