NOVELTY - Preparation of polyolefin involves introducing at least one monomer, catalyst and diluent into olefin polymerization zone (I) to form polyolefin; and introducing at least one catalyst deactivating agent (II) into (I) whereby polymerization is stopped or its rate is slowed; and restarting polymerization by introducing at least one catalyst in (I). (I) Comprises slurry of polymerization reactor that is loop reactor or stirred tank reactor.    USE - In the preparation of polyolefin (claimed).    ADVANTAGE - The method minimizes capital, maintenance and energy costs of polyolefin production by eliminating need for silos and their associated equipments or by reducing the costs associated with temporarily stopping or slowing polyolefin production. The method requires minimum time to restart as it can be restarted without the use of scavenger to remove poison from the reactor. The use of (II) allows method to shut down polyolefin production, thus minimizing amount of polyolefin produced that do not meet quality specification. The method avoids use of storage silos and related equipment, thus providing cost savings.    DETAILED DESCRIPTION - An INDEPENDENT CLAIM is included for olefin polymerization apparatus comprising a slurry polymerization reactor suitable for polymerizing at least one monomer in the presence of a catalyst, diluent to form polyolefin. The reactor comprises at least one effluent removal conduit for removing effluent containing polyolefin; a supply of (II) operatively connected to the reactor so that (II) can be introduced into the reactor at selected time and in selected quantities; device for determining the quantity of catalyst in the reactor; a separation zone operatively connected to the effluent removal conduit and capable of separating the effluent into polyolefin lean stream or polyolefin rich stream, where the separating zone comprises at least one polyolefin rich stream removal conduit; and an agglomerating zone operatively connected to the polyolefin rich stream removal conduit and capable of agglomerating polyolefin from polyolefin rich stream.