Process for producing a propylene homopolymer, a propylene random copolymer or a heterophasic propylene polymer, which process comprises the steps of (a) prepolymerisation, (b) slurry phase polymerisation (c) gas phase polymerisation and optionally (d) one or two further gas phase polymerisation(s), whereby the process is performed in the presence of a catalyst system comprising (x) a Ziegler-Natta procatalyst and (y) an organometallic cocatalyst and (z) an external donor represented by formula (II) and/or (III) Si(OCH 2 CH 3 ) 3 (NR 1 R 2 ) (II) or R 3 nR 4 mSi(OR 5 )z (III) wherein R 1 and R 2 can be the same or different and represent a hydrocarbon group having 1 to 12 carbon atoms and R 3 and R 4 can be the same or different and represent a hydrocarbon group having 1 to 12 carbon atoms, R 5 is methyl or ethyl, z is 2 or 3, preferably 2, m is 0 or 1, n is 0 or 1, with the proviso that n + m +z = 4, the external donor being added in an amount of 3 to 30 wt%, based on the total amount of external donor added, to the prepolymerisation step and in an amount of 70 to 97 wt%, based on the total amount of external donor added, to the slurry polymerisation step.