Preparing propylene polymer compositions comprising 50 to 90% by weight of a propylene (co)polymer, and 10 to 50% by weight of an ethylene copolymer. The polymerization process includes, in the presence of a catalyst system: (i) a first step of polymerizing propylene to produce the propylene (co)polymer; and (ii) a successive gas-phase polymerization step performed in the presence of the propylene (co)polymer, ethylene, and one or more α-olefins to produce the ethylene copolymer. The catalyst system contains the product obtained by contacting: (a) a solid catalyst component comprising at least two internal electron donor compounds, a succinate and a 1,3-diether; (b) an aluminum hydrocarbyl compound, and (c) optionally, an external electron donor compound.