I’d certainly say “du”. as in “Ce journaliste du Monde a été arrêté dans des circonstances encore inconnues”. “De Le Monde” would sound ugly. Like some automated voice messages heard in train stations: “Le train dessert la gare de Le Mans”. The computed program does not detect the part of the name of the city that is actually an article.

Actually it’s a pretty tough question as there is no proper answer to it except “common sense.” In the majority of cases we would contract the article.

When writing we can also use forms to avoid having to make this decision, for example by specifying that “Le Monde” is a newspaper Voici la dernière notice du journal Le Monde

It’s mainly by the appreciation of the writer/translator. I’m working as translator and in my company we tend to specify the nature of the product between the contraction and its name,as I did for du journal Le Monde

However, we never make contractions when talking about a person (de Le Corbusier).

And as Jiliagre stated, it’s a duplicate of Contraction de « de Les », « de La » ou « de Le » dans les noms propres, I still answer so you have english translation of this french thread.