Is it always possible to reboot the phone without removing the battery? I've had two Android phone up til now (HTC Wildfire and SGS2). Sometimes they crashed and the only way to get them restart was to remove the battery. Now I'm thinking about buying a Sony Z1 Compact. Removing the battery is no longer something you can do without tools, and looking at youtube videos I think I will never do this.

Now I'm wondering - in those cases where a phone does not react anymore, how can you reboot the phone if you cannot remove the battery? Waiting until the batter is dead is one option, but that could take a day or longer, and is no option either. Or was I too impatient and is it just a matter of waiting? 

In short: Is it always possible to reboot the phone without removing the battery?

There are many phones out there with a fixed battery, so I'm not the first with this problem. What are your experiences?
As pointed out by my other answer, and confirmed by the OP for the given device with reference to a blog article on the Z1, most devices with non-removable batteries have other means to "disconnect from power", usually "reset buttons" or "reset holes" to be poked with a pen or similar object. This should remedy in situations where the device freezes in a way not even the power button works. But it still leaves the disadvantage of not being able to "switch" the battery to a backup should you e.g. run out of power, of course.