Should I use an article with the word “part”?

I’ve noticed that some people use an article with the word “part” and the bulk of them does not (at least at some web pages I skimmed through). Which is the correct variant? I think that parts of something are countable, so there should be an article, whether definite or not.

A few examples right out of my head. Please, do not consider it as meaningful sentences. These are just for demonstrating and mean almost nothing.

KDE is a part of the Linux GUI.
The mentioned district is a part of the city.
This stick was a part of that tree.
Something is the main part of his success.

Here you go – a few real life examples I’ve just stumbled with:

… the resulting moc file is included in a _automoc.cpp file, which is then compiled as part of the target.

DarwinPorts was started in 2002 as part of the OpenDarwin project.

It was part of Tax Day protests held throughout the 1990s and earlier.

Solution 1:

All the sentences with X is/was a part of Y do not require the indefinite article.

KDE is part of the Linux GUI.
The red-light district is part of the city.
This stick was part of that tree.

are all fine without a.

X is *{the/a} main part of his success.*

however, requires the definite or indefinite article. OTOH, it's not a great sentence.

X is the main reason for his success.

is much better and more natural.

If you're talking about a long novel, say, you might be able to say:

Chapters 10-20, which depict in great detail the heroine's biography and the reasons for her unusual personality and character, are {the/a} main part of the novel.

The article is required here because there's a adjective in front of the noun. The other sentences don't include that adjective before part. The sentence

This is main part of the novel

is ungrammatical.