The disclosure relates to a process for polymerising olefins in multi stage polymerisation process configuration, comprising a) polymerising in a first polymerisation step first olefin monomer, optionally in the presence of at least one other alpha olefin monomer, in the presence of a polymerisation catalyst so as to form a first polymer component (A), and b) polymerising in a second polymerisation step in gas phase second olefin monomer, optionally in the presence of at least one other alpha olefin comonomer, in the presence of the first polymer component (A) of step a) and an induced swelling agent, so as to for a second polymer component (B), wherein the first polymer component (A) and the second polymer component (B) are produced at production rates meeting a predetermined target weight ratio of the second polymer component (B) to the first polymer component (A), the process comprising the steps of: i) determining a first weight ratio of the second polymer component (B) to the first polymer component (A) in the second polymerisation step, and ii) increasing the concentration of the induced swelling agent in the second polymerisation step if the determined first weight ratio is less than the predetermined target weight ratio, or iii) decreasing the concentration of the induced swelling agent in the second polymerisation step if the determined first weight ratio is greater than the predetermined target weight ratio, or iv) maintaining the concentration of the induced swelling agent in the second polymerisation step if the determined first weight ratio equals the predetermined target weight ratio. The disclosure further relates to use of an induced swelling agent in a gas phase polymerisation step for improving gas phase production split in a multi-stage olefin polymerisation process.