"How can a good God condem man to hell?"
First of all, the goodness and justice of God are two attributes which in reality are one. The good God must also be just because if he is not just he cannot be good.
What do we mean by the Justice of God?
God created man to worship and serve him and to participate in his heavely glory. Man sin against God brought about physical and spiritual death. Some might think that this is unfair but consider: God is a being of infinite holyness, majesty and glory who derserves absolute obedience. The revolt of a mere creature deserves infinite punishment. Christ death on the Cross shows us the seriousness of sin and this satisfied the justice of God. We are justified by the death of Christ. This is the way of God's Justice and Mercy. But in the nature of things, man must believe in Christ and obey Him to be saved. Man nature reamains free to accept or reject God's salvation.
Does God send anyone to Hell?
Pope John Paul II states: "Eternal damnation", ... is not attributed to God's initiative because in his merciful love he can only desire the salvation of the beings he created. In reality, it is the creature who closes himself to his love. Damnation consists precisely in definitive separation from God, freely chosen by the human person and confirmed with death that seals his choice for ever. God's judgement ratifies this state."
Of course, God created the structure of universal reality forseeing that some angels and men would revolt against his holyness and majesty and in this sense God created hell. To make clear what we mean by "structure of universal reality" let us suppose that man overeats and suffers a stomach ache. Who caused the stomach ache? Man who overate but the pain he experiences is also God's judgement because He created man's stomack with limitations. So it is also God's judgment that any man who revolts against his dependence on God and the purpose for which he was created, should suffer the inevitable spiritual death and the sufferings this entails.
In Dante Inferno, Canto 3: 4-9 we read:
"JUSTICE MOVED MY MAKER CELESTIAL;
I WAS CREATED BY THE DIVINE POWER,
BY THE SUPREME WISDOM, AND BY LOVE PRIMEVAL. *
ONLY ETERNAL THINGS ARE OLDER *
THAN I; AND I WILL FOREVER ENDURE.
ABANDON EVERY HOPE, YOU WHO ENTER.
Praise God for His Works