a method and apparatus are disclosed for producing polyolefins by a fluidized gas phase polymerization process utilizing two stacked , substantially vertically aligned , reactors and a substantially vertical product transfer line for transfer of intermediate polyolefin polymer product from the polymerization zone of the upper reactor to the polymerization zone of the lower reactor . this abstract shall not be construed to define or limit in any way the scope of the invention , which is measured solely by the appended claims .