Abstract: 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(OCH2CH3)3(NR1R2) (II) or R3nR4mSi(OR5)z (III) wherein R1 and R2 can be the same or different and represent a hydrocarbon group having 1 to 12 carbon atoms and R3 and R4 can be the same or different and represent a hydrocarbon group having 1 to 12 carbon atoms, R5 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 prepolymerisatio