The Samsung Galaxy S6 and other new Galaxy devices as Galaxy S7 and maybe Galaxy S8 don’t include a removable battery.
So what to do when your phone suddenly locks up?

Well you still have a somewhat less common way to do force a restart or reboot.

At any point, press the Power/Lock button and the Volume down button at the same time and hold it down for 10-20 seconds.

This will have the same effect as removing the battery on older devices would have had.