The following warning occurs while the simulation netlist is being generated:
"WARNING:NetListWriters:518 - Design does not contain multiple hierarchical blocks with KEEP_HIERARCHY property. Design will be written to a single file. Completed process "Generate Post-Place & Route Simulation Model"."
This warning indicates that you have turned on the option to Generate Multiple Hierarchical Netlist Files in the Simulation Model Properties, but hierarchy was not maintained in the design, thus preventing NETGEN from being able to generate multiple hierarchical netlist files.
There are several reasons why hierarchy might not have been maintained in the design. This most likely reason is that hierarchy was not maintained during Synthesis. Please refer to (Xilinx Answer 17693) for information on how to preserve hierarchy during Synthesis and Implementation.