A system for producing polyolefin, comprising a gas phase reactor for polymerizing an olefin to obtain polymerization product, the gas phase reactor comprising - a gas distribution plate arranged inside the gas phase reactor; - a first outlet for continuously withdrawing polymerization product from the gas phase reactor as a first product stream, the first outlet being arranged above the gas distribution plate; and - a second outlet for continuously withdrawing polymerization product from the gas phase reactor as a second product stream, the second outlet being arranged above the gas distribution plate; wherein the system further comprises - a first outlet tank in fluid communication with the first outlet via a first passage, wherein the first passage comprises a first valve means for controlling the flow of the first product stream in the first passage and wherein the first outlet tank is arranged to receive the first product stream and to concentrate the first product stream; - a product receiver tank in fluid communication with the second outlet via a second passage, wherein the second passage comprises a second valve means for controlling the flow of the second product stream in the second passage, and wherein the product receiver tank is arranged to receive the second product stream; and - a control means in communication with the first valve means and the second valve means and arranged to control the operation of the first valve means and the second valve means so that flow in only one of the first passage and the second passage is allowed at a time; and wherein the second passage comprises a first heater arranged to heat the second product stream. - a gas distribution plate arranged inside the gas phase reactor; - a first outlet for continuously withdrawing polymerization product from the gas phase reactor as a first product stream, the first outlet being arranged above the gas distribution plate; and - a second outlet for continuously withdrawing polymerization product from the gas phase reactor as a second product stream, the second outlet being arranged above the gas distribution plate; wherein the system further comprises - a first outlet tank in fluid communication with the first outlet via a first passage, wherein the first passage comprises a first valve means for controlling the flow of the first product stream in the first passage and wherein the first outlet tank is arranged to receive the first product stream and to concentrate the first product stream; - a product receiver tank in fluid communication with the second outlet via a second passage, wherein the second passage comprises a second valve means for controlling the flow of the second product stream in the second passage, and wherein the product receiver tank is arranged to receive the second product stream; and - a control means in communication with the first valve means and the second valve means and arranged to control the operation of the first valve means and the second valve means so that flow in only one of the first passage and the second passage is allowed at a time; and - a first outlet tank in fluid communication with the first outlet via a first passage, wherein the first passage comprises a first valve means for controlling the flow of the first product stream in the first passage and wherein the first outlet tank is arranged to receive the first product stream and to concentrate the first product stream; - a product receiver tank in fluid communication with the second outlet via a second passage, wherein the second passage comprises a second valve means for controlling the flow of the second product stream in the second passage, and wherein the product receiver tank is arranged to receive the second product stream; and - a control means in communication with the first valve means and the second valve means and arranged to control the operation of the first valve means and the second valve means so that flow in only one of the first passage and the second passage is allowed at a time; and wherein the second passage comprises a first heater arranged to heat the second product stream.