Comment 48 for bug 1464917

Have you tried @Adam idea turn your swap off just prior to shutdown, allow the command to process its a work around that seems to work for many. It may take 10+ minutes if a lot of data was being held in swap but it should shutdown.
sudo swapoff -a && systemctl poweroff
The amount of data held in swap must be lower than the amount available in Ram or the command will fail FYI.