This is a theological topic that is vast and extensive. Is it possible for a non-Shi'a to enter heaven.
In general, Shi'a Ulama do not condemn all non-Shi'ah to Hell. There might be some people who were not informed in any way about the true path of Islam and the true path of Ahlul Bayt (a.s.).
If there is someone who hates Ahlul Bayt (a.s.), and shows hostility to them, they are certainly condemned to Hell.
If there is someone who knows the true status of Ahlul Bayt (a.s.), and out of stubbornity refuses to follow the truth, then of course their judgment will be harsh.
The important thing to mention in this topic is that as Muslims we must stay away from the Takfeeri mentality, and treat all Muslims equally in this Dunya. As for what happens in the Akhirah, that is with Allah ta'ala.
And Allah knows best.