I recently got my American citizenship and in the ceremony, they "trade-in" your Green Card in exchange for the certificate. With that, you can then request a passport. But I had to leave the country immediately due work trip.
I was able to request the passport once I arrived in my country, but I am still waiting for it.
I was wondering if it is possible to get into the USA holding my country's passport plus the American Naturalization certificate?
If your other passport is issued by Canada or Bermuda, yes. If it is issued by a VWP country, maybe: in that case you will need a valid ESTA authorization before you will be able to board a flight to the United States with that passport, and there are possible complications even if you have a valid ESTA, so if this is your situation especially if you are traveling through a preclearance airport. There are several questions about this, mostly over at Travel.
If you have a VWP passport without an ESTA or any other country's passport, you should be able to enter by land from Canada or Mexico.
(Canadians and Bermudians can travel to the US for tourism without a visa and without ESTA, so you can board a flight to the US with a passport from one of those countries. Travelers using VWP-eligible passports need ESTA approval, but there are reports of US citizens receiving ESTA approval despite disclosing their US citizenship in the application. The regulations don't say anything one way or the other. The regulations are quite explicit, by contrast, in saying that US citizens are not to be issued visas, so you won't be able to get one, and if you have a passport that requires a visa you won't be able to board a flight to the US using that passport without a visa.)
If you manage to make it to a US port of entry, and you can satisfactorily prove your US citizenship (e.g. with a US Certificate of Naturalization with a photo ID like your foreign passport), then you cannot be denied entry, although they may temporarily detain you to give you a lecture and verify your citizenship.
However, the problem is how to make it to a US port of entry. If you are in (or can get to) Canada or Mexico, you can enter the US by land, and get to the US port of entry directly with no problem. But if you must fly to the US, then the airline is required to deny you boarding unless you have certain acceptable documents for travel to the US (as described in CBP's carrier information guide), and a Certificate of Naturalization is not an acceptable document. And as phoog's answer describes, your foreign passport by itself is also not sufficient, unless it has a US visa, or ESTA (if you are a Visa Waiver Program national), or the passport is from one of the few countries that are visa-exempt to the US without the Visa Waiver Program.
