A method for controlling a slurry polymerization for the preparation of polyethylene, where the polyethylene is formed in a polymerization reactor comprising, as slurry, a suspension of particulate polyethylene in a suspension medium comprising a diluent by contacting a Ziegler-type catalyst, ethylene, and either hydrogen or, as comonomer(s), one or more C 3 to C 10 alpha-olefins or hydrogen and one or more C 3 to C 10 alpha-olefins at a reactor temperature from 60°C to 95°C and a reactor pressure from 0.15 MPa to 3 MPa, where a diluent, ethylene, the Ziegler-type catalyst, and either hydrogen or one or more C 3 to C 10 alpha-olefins or hydrogen and one or more C 3 to C 10 alpha-olefins are fed to the polymerization reactor and slurry is withdrawn from the polymerization reactor, and where the polymerization reactor has a reactor slurry section comprising the slurry and a reactor vapor section comprising a vapor comprising ethylene, diluent, and either hydrogen or one or more C 3 to C 10 alpha-olefins or hydrogen and one or more C 3 to C 10 alpha-olefins, the method comprising: a) withdrawing vapor from the reactor vapor section; b) analyzing the vapor to determine its composition; and c) calculating thereof, based on the pressure within the polymerization reactor, values for the partial pressure of ethylene, for the partial pressure ratio of hydrogen to ethylene and for the partial pressure ratios of the one or more comonomers to ethylene; d) maintaining the flow rate of the ethylene to the polymerization reactor at a constant level; e) adjusting the flow rate of the Ziegler-type catalyst to the polymerization reactor to maintain the ethylene partial pressure at a targeted ethylene partial pressure; and f) either adjusting the flow rate of hydrogen to the polymerization reactor to maintain the hydrogen/ethylene partial pressure ratio at a targeted hydrogen/ethylene partial pressure ratio; or adjusting the flow rates of the one or more comonomers to the polymerization reactor to maintain the comonomer/ethylene partial pressure ratios at targeted comonomer/ethylene partial pressure ratios; or adjusting both the flow rate of hydrogen to the polymerization reactor to maintain the hydrogen/ethylene partial pressure ratio at a targeted hydrogen/ethylene partial pressure ratio and adjusting the flow rates of the one or more comonomers to the polymerization reactor to maintain the comonomer/ethylene partial pressure ratios at targeted comonomer/ethylene partial pressure ratios. a) withdrawing vapor from the reactor vapor section; b) analyzing the vapor to determine its composition; and c) calculating thereof, based on the pressure within the polymerization reactor, values for the partial pressure of ethylene, for the partial pressure ratio of hydrogen to ethylene and for the partial pressure ratios of the one or more comonomers to ethylene; d) maintaining the flow rate of the ethylene to the polymerization reactor at a constant level; e) adjusting the flow rate of the Ziegler-type catalyst to the polymerization reactor to maintain the ethylene partial pressure at a targeted ethylene partial pressure; and f) either adjusting the flow rate of hydrogen to the polymerization reactor to maintain the hydrogen/ethylene partial pressure ratio at a targeted hydrogen/ethylene partial pressure ratio;