It follows from the error message, that you somehow submit the entire string Database["DB2"].ExecuteNonQuery("call SYSPROC.ADMIN_CMD ('REORG TABLE DB2ADMIN.XCATENTRYEXT index CATENTRY_ID INPLACE')")
as a SQL statement, which obviously is incorrect.
Simply issue these on the shell command line:
db2 connect to <your database name here>
db2 REORG TABLE DB2ADMIN.XCATENTRYEXT