Method for polymerization of olefin

A method for polymerizing at least one olefin comprising:

• reacting said olefin in a first reactor connected in series to one or more reactors, said one or more reactors comprising at least one secondary gas-phase reactor, thereby forming a mixture comprising a polymer and a multi-component gas;
• conveying said mixture into one or more reactors comprising at least one secondary gas-phase reactor;
• reacting said mixture in said one or more reactors;
• removing said multi-component gas from said secondary gas-phase reactor;
• liquefying a portion of said multi-component gas, thereby forming a mixture comprising a first gas component and a first liquified gas component;
• returning said first liquified gas component to the same said secondary gas-phase reactor from which said multi-component gas was removed;
• liquefying at least a portion of said first gas component, thereby forming a mixture comprising a second gas component and a second liquified gas component;
• discharging at least a portion of said second gas component;
• and
• returning said second liquified gas component to the same said secondary gas-phase reactor from which said multi-component gas was removed;
• wherein said one or more reactors are connected in series to each other and said first reactor, and said liquefying is by pressurizing and/or cooling said multi-component gas.